1. Field of the Invention
The present invention relates to an image pickup device and an image pickup method for shooting an object in a wide dynamic range by combining images having two types of exposure times.
2. Description of the Related Art
In a related image pickup device using a solid-state image pickup element, such as a charge coupled device (CCD), a light quantity (exposure amount) input to the image pickup element is adjusted by a stop or on the basis of an electronic shutter speed. That is, when a bright scene is picked up, in order to prevent what is called overexposure (in which an output signal of the image pickup element is saturated) from occurring, the exposure amount is reduced. In contrast, for a dark scene, in order to prevent what is called underexposure from occurring, the exposure amount is adjusted to a large exposure amount.
However, when a scene 5 having a large difference in brightness is picked up (that is, when a backlight image pickup operation is performed or indoor and outdoor pickup operations are simultaneously performed), since a dynamic range of the solid-state image pickup element that is used is insufficient, merely adjusting the exposure amount results in overexposure caused by saturation of the bright portion, or underexposure at the dark portion. Therefore, both portions cannot be properly reproduced.
For overcoming this problem, the following image pickup device is available. In the image pickup device, on the basis of a short-time exposure image and a long-time exposure image obtained by an image pickup section, after performing a gain operation or an offset operation on the short-time exposure image, the short-time exposure image is combined with the long-time exposure image. Then, a level compression operation is performed on the combined image, to obtain a combined image in a wide dynamic range.
For example, the following image pickup devices are proposed (refer to, for example, Japanese Unexamined Patent Application Publication Nos. 2004-56573 and 2004-120205). In one of the image pickup devices, when the short-time exposure image and the long-time exposure image are combined with each other, and the combined image is compressed to generate an image in a wide dynamic range, the short-time exposure image is subjected to multiple addition operations for a few fields. This image subjected to the multiple addition operations is combined with the long-time exposure image as an intermediate-time exposure image, to generate an image in a wide dynamic range. This makes it possible to effectively prevent a reduction in the S/N ratio at an intermediate input level. In the other image pickup device, dynamic ranges assigned to the respective long-time exposure image and short-time exposure image are dynamically changed using a histogram, to reproduce an image under optimal conditions, such as with optical gradation and exposure.
Further, the following image processing device is proposed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2005-204195). In the image processing device, for changing gradation in accordance with a change in brightness for each frame of a dynamic image, brightness area information that represents the width of a brightness distribution of an image is calculated. On the basis of smoothed brightness area information in which the calculated brightness area information is smoothed in times-series, the brightness of the image is normalized in accordance with a brightness area of a display device that displays the image.